The long-term objective of this series of studies is to determine the factors which relate cell volume to transport activity in the living epithelial cells of the frog urinary bladder. Cell volume can be followed repeatedly by the method described below, and will be studied before and after (a) the osmolality of the bathing media is altered in the presence and absence of antidiuretic hormone, (b) the ionic oncstituetns of either or both media are varied at constant osmolality, (c) transport is inhibited with ouabain or amiloride or stimulated with vasopressin or aldosterone, and (d) application of transepithelial voltage and current clamps. The tissue is placed in a chamber designed to allow good optics as well as the ability to measure transepithelial electrical events. The cells are viewed with a microscope and the images projected with a television camera onto a video monitor. With the aid of a computer and a stepper motor, optical sections are obtained rapidly as the microscope is focused through the tissue, and at each level (0.5 or 1.0 mum per step) a video imaae is recorded on tape. Subsequently, the images are played back, and at each depth a computer-driven cursor is used to trace a particular cell on the video screen. From the coordinates of the cursor positions the area of each section is calculated, and from this information and the known distance between sections, the volume may be reconstructed. Since the major function of epithelial tissues is to transport ions and water, and hence to maintain homeostasis, and since there is some suggestion that the maintenance of cell volume is critical for normal cell function, it would seem important to study the relationship between transport and volume. However, very little is known about this relationship, because until recently it has been impossible to measure cell volume in a non-destructive and repeatable fashion. In these studies, such measurements are feasible for the first time.